Conventionally, tire condition monitoring apparatuses have been known that monitor the condition of a tire such as the air pressure of the tire. A tire valve unit used in such a tire condition monitoring apparatus includes a tire valve and a sensor unit, which is secured to the tire valve. The tire valve unit is mounted on a wheel by press-fitting the tire valve into a mounting hole in the wheel (for example, see Patent Document 1).
In the above-mentioned tire valve unit, as shown in FIG. 10, a sensor unit 101 includes a housing 102, which accommodates a sensor that detects a condition of the tire. The housing 102 also includes a fitting groove 103 in which a nut 104 is embedded.
The tire valve 110 includes a tubular valve stem 111 and a rubber body 112, which is located on the outer circumferential surface of the valve stem 111. Part of the valve stem 111 projects from the body 112. A coupling portion 113, which projects from the body 112, includes an insertion hole 114, which extends through the coupling portion 113 in a radial direction. A bolt 115, which is inserted in the insertion hole 114, is screwed to the nut 104 to integrate the tire valve 110 with the sensor unit 101.
The end of the tire valve 110 in the axial direction to which the sensor unit 101 is not secured is referred to as a first axial end 110a. The tire valve 110 is inserted in a mounting hole 121 in a wheel 120 from the first axial end 110a to mount the tire valve unit on the wheel 120. When part of the body 112 of the tire valve 110 the outer diameter of which is greater than the diameter of the mounting hole 121 reaches the mounting hole 121, the body 112 is elastically deformed in a stretching manner in the axial direction of the tire valve 110. Since the tire valve 110 is inserted in the mounting hole 121 from the first axial end 110a, reaction force toward a second axial end 110b acts on the tire valve 110 from the peripheral portion of the mounting hole 121 in the wheel 120. Thus, part of the body 112 located inside the wheel 120 is elastically deformed in a stretching manner toward the second axial end 110b. 
If the body 112 and the sensor unit 101 are in close contact with each other, the elastic deformation of the body 112 is prevented by the sensor unit 101, and the tire valve unit may be hard to mount on the wheel 120. Thus, in the tire valve unit disclosed in Patent Document 1, a gap 130 is provided between the body 112 and the housing 102 of the sensor unit 101 to keep a space for allowing for elastic deformation of the body 112.